1. Field of the Invention
This invention relates generally to the field of motors, and more particularly, to circuits and methods for determining the position of a brushless DC motor's rotor.
2. Description of the Related Art
Brushless DC motors generally comprise a stator and a permanent magnet rotor. The stator includes at least one coil; an excitation voltage is periodically applied across the coils, and the resulting electromagnetic field causes the rotor to rotate with respect to the stator.
To ensure that the application of the excitation voltage is correctly timed, it is necessary to know the position of the rotor with respect to the stator coils. This is conventionally accomplished with the use of one or more Hall effect sensors, with the stator coils activated by drive electronics that are cued by signals received from the sensors. One motor system of this type is illustrated in FIG. 1. Here, a motor 10 includes stator coils 12 and 14 and a Hall sensor 16. A controller 18 operates switches 20 and 22, here bipolar transistors, to periodically apply an excitation voltage VDD across coils 12 and 14, respectively. The position of the rotor is sensed with Hall sensor 16, and reported to controller 18 via an amplifier 20; with this position information, controller 18 can operate switches 20 and 22 as necessary to maintain the rotation of the motor's rotor, and to control its speed if desired.
However, the approach depicted in FIG. 1 has a number of drawbacks. The need for Hall sensors contributes a significant cost to the motor. In addition, the sensors must be placed within the motor along with other electronics, which limits the design of the motor structure, and may degrade its reliability.
One alternative to the use of Hall sensors is employed in some multiple-phase DC brushless motors. Here, the electro-motive force (EMF) generated in a passive coil while another coil is energized is measured and used to determine the position of the rotor.